Semiconductor processing for forming integrated circuits requires a series of processing steps. These processing steps include the deposition and patterning of material layers such as insulating layers, polysilicon layers, and metal layers. The material layers are typically patterned using a photoresist layer that is patterned over the material layer using a photomask or reticle. Typically, the photomask has alignment targets or keys that are aligned to fiduciary marks formed in the previous layer on the substrate. However, as the size of integrated circuit features continues to decrease, it becomes increasingly difficult to measure the overlay accuracy of one masking level to the previous level. This overlay metrology problem becomes particularly difficult at submicrometer feature sizes where overlay alignment tolerances are reduced to provide reliable semiconductor devices.
One type of overlay measurement that has recently been developed is based on the diffraction of light by a number of alignment pads. FIG. 1 shows one example of a diffraction based overlay measurement pattern 10. As illustrated in FIG. 1, pattern 10 includes two separate alignment pads 20 and 30. The pads are made of the materials that are deposited onto the substrate. Each pad 20, 30 includes a bottom layer 12, a bottom diffraction grating 24, 34, a top layer 16, and a top diffraction grating 28, 38. The bottom layer 12 may be, e.g., one or more films on a substrate. The bottom diffraction gratings 24, 34 may be, e.g., metal lines surrounded by dielectric material. The top layer 16 may be, e.g., one or more films disposed over the substrate and the bottom diffraction grating material. The top diffraction gratings 28, 38 may be, e.g., metal lines, metal lines surrounded by dielectric material, photoresist lines, etc.
As illustrated in FIG. 1, the top diffraction gratings 28, 38 are intentionally offset from the bottom diffraction gratings 24, 34. In addition, the offsets of pads 20 and 30 are equal in magnitude but opposite in direction. With such an offset configuration, a misalignment to the left or right in FIG. 1 can be easily detected using pattern 10.
The diffraction based metrology pattern 10 relies on the symmetry of the alignment pads 20 and 30. When the symmetry is broken by an alignment error, the diffraction patterns produced by each alignment pad will change by different amounts. FIG. 2, by way of example, illustrates metrology pattern 10 with an alignment error to the right in the figure, which as can be seen, creates an asymmetry between the pads 20 and 30. By comparing the diffraction patterns from pads 20 and 30, the amount of alignment error can be determined.
In the presence of local process variations, however, diffraction based overlay measurement may be biased. Local process variations are created during processing of the measurement pattern 10, e.g., during chemical mechanical polishing, and result in differences in film thickness, grating height or grating linewidth. Local process variations may cause an asymmetry between the alignment pads 20 and 30 even if there is no alignment error. Accordingly, when there is an alignment error, the difference in collected spectra from pads 20 and 30 contains information from both the overlay shift and the process variations. These contributions cannot be directly separated.
Thus, what is needed is an improved diffraction based metrology technique that can correct for local process variations when measuring the overlay error.